A low carbon resulfurized free-machining steel is a versatile steel material which is widely used for hydraulic components in automobile transmission and other small parts such as screws and printer shafts which do not require particularly high strength. When an improved roughness of the finished surface after machining and ease of chip disposability are required, a lead sulfur free-machining steel produced by adding lead (Pb) to the low carbon resulfurized free-machining steel is used.
Pb in the free-machining steel is quite effective in improving machinability of the steel. This Pb, however, has been pointed out to be an element which is toxic to the human body, and Pb is also associated with various other problems including lead fumes in steelmaking and chip disposability. In view of such situations, there is a strong demand for a free-machining steel which has realized a practical machinability without adding Pb.
With regard to such low carbon resulfurized free-machining steel, various techniques have been proposed for realizing a Pb-free steel having an improved machinability. For example, Patent Document 1 proposes a technique in which the machinability (roughness of the finished surface and easiness in disposing of the chips) has been improved by controlling size of the sulfide inclusion. Patent Document 2 discloses importance of controlling oxygen content in the steel for controlling the size of the sulfide inclusion. Also proposed are techniques of improving machinability by controlling oxide inclusion in the steel (see, for example, Patent Documents 3 to 5).
In the meanwhile, also proposed are techniques of improving the machinability by adequately controlling the chemical composition of the steel material (See for example, Patent Documents 6 to 9).
The techniques that have been proposed are useful in view of improving the machinability of the free-machining steel. The steel produced by these techniques, however, did not have the favorable machinability of the level comparable to the Pb-containing steel, in particular, in the roughness of the finished surface after the forming process.
In addition to the machinability as described above, it is also important that the Pb-free steel also has a good productivity. In this view, the steel should also be capable of being produced by continuous casting with no occurrence of surface defects and the steel also needs to be capable of rolling. However, such continuous casting process has been said to be disadvantageous in producing a steel having a good machinability. Therefore, it is also important to provide a free-machining steel with a good machinability which can be produced by the continuous casting.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-253390    [Patent Document 2] Japanese Patent Laid-Open No. 1997-31522    [Patent Document 3] Japanese Patent Laid-Open No. 1995-173574    [Patent Document 4] Japanese Patent Laid-Open No. 1997-71838    [Patent Document 5] Japanese Patent Laid-Open No. 1999-158781    [Patent Document 6] Japanese Patent Laid-Open No. 2000-319753    [Patent Document 7] Japanese Patent Laid-Open No. 2001-152281    [Patent Document 8] Japanese Patent Laid-Open No. 2001-152282    [Patent Document 9] Japanese Patent Laid-Open No. 2001-152283